I read an article the other day from ArchDaily on calculating the thermal transmittance of a wall (see link below). It occurred to me to write a post on how Revit can calculate thermal resistance - R-value - of materials and assemblies... here it is.
ArchDaily article:
How to Calculate the Thermal Transmittance (U-Value) in the Envelope of a Building
By way of comparison, let's look at the table "Calculating Assembly Wall R-Value" from ColoradoEnergy.Org found here: http://www.coloradoenergy.org/procorner/stuff/r-values.htm
*This image is posted by permission from Randy L. Martin
The referenced graph has a column calculating the stud-only condition and another for insulation-only, filling the 3.5" space in the wall. Let's look at each condition in Revit:
However, the chart is dealing with a 2x4 (3 1/2") stud, not a 2x6 (5 1/2") so let's change that in...
Revit (see image below). Notice the R-value has automatically changed. R = 5.90
If we delete all layers except one, in a given wall assembly, we get an R-value based on that material's thickness, which we can use to compare to the linked chart. R = 0.68
The chart below lists the component R-values and then a total. We should understand there is a big hole with the default wall type... it assumes a continuous/solid 3 1/2" wood layer with no space or insulation! However, this closely matches the "studs" column in our example chart.
The one exception being the absence of the outside/inside air film layers. You don’t need to add these for Autodesk Insight or any dynamic thermal simulation like DOE2 or EnergyPlus because these are calculated throughout the simulation e.g. accounts for the relationship between increasing wind speed and reducing exterior convection coefficient.
The thermal properties for gypsum board appears to be off, as several sources I have checked list an R-value of 0.45 for a 1/2" thickness. However, for comparative analysis this does not really matter. But you can change this per the example shown later in this post.
It is worth pointing out that membrane layers are not included even though they have thermal properties assigned in Revit, as zero times anything is zero!
As just shown, Revit handles the calculation of resistance/conductance from a purely first principles perspective i.e.:
A search for proper thermal properties for fiberglass insulation...
We need to convert to the correct units... I used CalculatorEdge, Thermal Calculators:
We can adjust the Thermal Assets in Revit when needed. In this case I enter the desired thermal conductivity:
Now we have a more realistic R-value... notice in our reference link that the next table down has a lower R-value for 3 1/2" batt insulation. R = 11.74
Here are the results for the insulation-only example:
If you do the math for the weighted studs (25%) v. insulation (75%) we end up with 11.55. If we factor in the outside/inside air films the result would be 12.40 - pretty close to the 12.03 in the referenced chart.
The resultant R-value is the same as the manual effort. R = 11.58
The triangle in Insight's dynamic input for Wall Construction represents the current BIM Setting, that is, the way it is currently modeled.
Super fun stuff!
For BIM Chapters updates, follow @DanStine_MN on Twitter
ArchDaily article:
How to Calculate the Thermal Transmittance (U-Value) in the Envelope of a Building
By way of comparison, let's look at the table "Calculating Assembly Wall R-Value" from ColoradoEnergy.Org found here: http://www.coloradoenergy.org/procorner/stuff/r-values.htm
*This image is posted by permission from Randy L. Martin
The referenced graph has a column calculating the stud-only condition and another for insulation-only, filling the 3.5" space in the wall. Let's look at each condition in Revit:
Stud-Only Calc
If we open the residential template that ships with Revit, we see the R-value listed for the wall type Exterior - Wood Siding on Wood Studs. R = 8.30
However, the chart is dealing with a 2x4 (3 1/2") stud, not a 2x6 (5 1/2") so let's change that in...
Revit (see image below). Notice the R-value has automatically changed. R = 5.90
If we delete all layers except one, in a given wall assembly, we get an R-value based on that material's thickness, which we can use to compare to the linked chart. R = 0.68
The chart below lists the component R-values and then a total. We should understand there is a big hole with the default wall type... it assumes a continuous/solid 3 1/2" wood layer with no space or insulation! However, this closely matches the "studs" column in our example chart.
The one exception being the absence of the outside/inside air film layers. You don’t need to add these for Autodesk Insight or any dynamic thermal simulation like DOE2 or EnergyPlus because these are calculated throughout the simulation e.g. accounts for the relationship between increasing wind speed and reducing exterior convection coefficient.
Component
|
R-Value
|
Outside Air Film
|
-
|
Exterior Wood Siding
|
0.90
|
½” Plywood Sheathing
|
0.68
|
3 1/2" Fiberglass
Batt Insulation
|
-
|
3 1/2" Stud
|
4.21
|
1/2" Drywall
|
0.11
|
Inside Air Film
|
-
|
Total
|
5.9
|
The thermal properties for gypsum board appears to be off, as several sources I have checked list an R-value of 0.45 for a 1/2" thickness. However, for comparative analysis this does not really matter. But you can change this per the example shown later in this post.
It is worth pointing out that membrane layers are not included even though they have thermal properties assigned in Revit, as zero times anything is zero!
- resistance = length / conductivity
- conductance = 1 / resistance
Insulation-Only Calc
Next, we need to look at the wall assembly with just insulation, no studs. We see that the result is way off due to the specific Thermal Asset (TA) used for batt insulation. The next couple images show how I did a quick internet search to find the correct Thermal Conductivity, did a required unit conversion and updated the TA. TIP: If you have access to WUFI Pro you can find accurate material properties in it's 'materials database.'
A search for proper thermal properties for fiberglass insulation...
We need to convert to the correct units... I used CalculatorEdge, Thermal Calculators:
Now we have a more realistic R-value... notice in our reference link that the next table down has a lower R-value for 3 1/2" batt insulation. R = 11.74
Here are the results for the insulation-only example:
Component
|
R-Value
|
Outside Air Film
|
-
|
Exterior Wood Siding
|
0.90
|
½” Plywood Sheathing
|
0.68
|
3 1/2" Fiberglass Batt Insulation
|
11.74
|
3 1/2" Stud
|
-
|
1/2" Drywall
|
0.11
|
Inside Air Film
|
-
|
Total
|
13.43
|
If you do the math for the weighted studs (25%) v. insulation (75%) we end up with 11.55. If we factor in the outside/inside air films the result would be 12.40 - pretty close to the 12.03 in the referenced chart.
Combining Studs and Insulation
From Revit Help:R-value A measure of the thermal resistance (or resistance to heat loss) through a material layer of a given thickness. R-value = Thickness / Conductivity.
Note: In cases where a specific material layer relates to an air gap, a material of non-uniform thickness (such as metal decking or an I beam) or combination of materials (such as a metal stud partition - air and steel) manual adjustments to layer thickness and/or thermal conductivity is required in order for the R-value to be calculated correctly. This is not to be confused with the R-value for an entire wall, floor, or roof element which is the sum of the resistances of each material layer that makes up each element.
Notice in the images below I created a custom Thermal Asset to account for the 25% Studs and 75% Insulation, with the total R-Value using weighted values for conductivity, specific heat and density.
The resultant R-value is the same as the manual effort. R = 11.58
Autodesk Insight
Now, as expected, we see the correct R-Value in Insight when Detailed Elements is checked in Revit's Advanced Energy Settings dialog.
The triangle in Insight's dynamic input for Wall Construction represents the current BIM Setting, that is, the way it is currently modeled.
Conclusion
While the thermal properties that ship with Revit may not be super reliable, just like the content they provide, it is a starting point and an example of how things can work. Of course there are several other challenges with assemblies, thermal by-pass and openings, but this is a good place to start for early energy modeling and comparative analysis.
BTW, thanks to Ian Molloy at Autodesk for answering a few questions I had related to this post!
BTW, thanks to Ian Molloy at Autodesk for answering a few questions I had related to this post!
Super fun stuff!
For BIM Chapters updates, follow @DanStine_MN on Twitter
